An ink-jet recording head is usable for printing color images by preparing ink of more than one color. However, it is essential to minimize the quantity of ink in the form of an ink droplet in order to reduce the size of each dot itself and to prevent ink from oozing out of the adjoining dots when an attempt is made to print images of substantially the same degree of print quality as photographs.
As Japanese Patent Publication No. Hei. 4-36071 discloses a method of technically forming very small dots by means of an ink-jet recording head through the steps of, as shown in FIG. 19, using a first signal S1 for rapidly expanding a pressure generating chamber so as to cause a meniscus to generate the Helmholtz resonance vibration by rapidly pulling back the meniscus from a nozzle opening, causing an ink droplet to be jetted by separating a part of the meniscus with kinetic energy originating from the energy of the Helmholtz resonance vibration, using a second signal S2 which maintains a substantially constant voltage for causing the meniscus to generate free vibration, and then using a third signal S3 for resetting the meniscus to a position where an ink droplet is properly jetted next time.
The aforementioned method will be described by reference to FIG. 20.
FIG. 20 shows a state of the meniscus after an ink droplet fit for printing is jetted because of the first signal S1 with the period Tc of the Helmholtz resonance vibration as a time unit, wherein a reference symbol M denotes the displacement of the meniscus on which the Helmholtz resonance vibration is superposed; and M', the displacement of the meniscus itself vibrated with an extremely long period Tm.
When the first signal S1 is set to a time period shorter than the period Tc of the Helmholtz resonance vibration, the Helmholtz resonance vibration is put in an active state of the Helmholtz resonance vibration, so that the Helmholtz resonance vibration with the period Tc is generated on the meniscus. This Helmholtz resonance vibration is generated in such a state that it has been superposed on the natural vibration M' of the meniscus displaced with the period Tm. When the natural vibration M' of the meniscus itself is brought close to the nozzle opening, a part of the meniscus is greatly swollen from the nozzle opening because of peaks of the Helmholtz resonance vibration P1', P2', P3' . . . and that part is isolated in the form of a very small ink droplet, that is, in the form of a satellite or an ink mist. The satellite or the ink mist conspicuously appears in an high-temperature environment as the viscosity of ink lowers.
An object of the present invention intended to solve the foregoing problems is to propose a method of driving an ink-jet recording head capable of discharging an ink droplet fit for the formation of a very small dot at a high driving frequency with the minimized quantity of ink without causing the generation of a very small useless ink droplet after the ink droplet is jetted.